1. Field of the Invention
The present invention relates to a current limiting circuit.
2. Discussion of the Related Art
Generally, two types of circuits for protecting a load against overcurrents are used. The most common protection method consists in disposing a clipping or breakover voltage limiting circuit in parallel with the load. However, in some cases, series circuits are desired. Such circuits are seldom used because, when they are fabricated in the form of semiconductor components, they generally require a current or voltage detection device and a separate control device. Therefore, such protection circuits are devices provided with a control terminal that has to be associated with rather complex control circuits. Two-terminal series components mainly include fuses or varistors. The drawback of fuses is that they must be replaced after each overload. Varistors are presently expensive components having a threshold that is not always precisely determined and is environment-dependent (more particularly, temperature-dependent). Additionally, a varistor unavoidably exhibits a thermal inertia and, for example, does not ensure protection against overcurrents occurring very shortly one after the other.
It has also been proposed to use depleted MOS transistors as current limiting components, the first terminal of the current limiting component being formed by interconnection of the gate and source terminals of the transistor, and the second terminal of the current limiting component being formed by the drain terminal of the transistor. This approach is for example described in U.S. Pat. No. 3,603,811 filed on Dec. 9, 1969. However, the above patent mentions this approach only to emphasize its drawbacks and, hitherto, such a device has not been implemented in any practical application.
The invention relates to a series protection circuit that is not intended for protection against a short in a load but against temporary overcurrents, for example, overcurrents that occur at the switching-on of a fluorescent lamp. The invention also applies when the protection threshold is comprised within a very large range, for example, from a few tens of milliamperes to several amperes.
The conventional drawing of a current limiting circuit is illustrated in FIG. 1; it merely consists of disposing in series with a load L a current limiting component 1. Of course, it is also desired that, when the current in load L has a normal value, the impedance of the current limiting component be as low as possible.